Metal articles that have long and thin portions and an equiaxed grain structure typically are cast with molds having gates placed at various locations along the length of the mold cavity. This gating is used to conduct molten metal which compensates for the decrease in the volume of the metal during solidification. The number of gates that are required depends upon the relationship between the length of the article being cast and the thickness of the article. It has been a common practice to provide gates which are spaced apart along the length of a mold by a distance of between 3 to 12 times the thickness of the article being cast. From ten to thirty-six gates would be required for an article having a maximum thickness of about 0.1 inch and a length of about 12 inches.
Gates promote the formation of defects in castings. For instance, hot tears and/or distortion tends to occur in positions of the cast article corresponding to gate locations in the mold. In addition, a stub usually remains at each gate location. These stubs must be removed, which is difficult to do when the cast article is curved. Another disadvantage associated with the use of gates is that an area of distinctly larger grain size is formed in the cast article at each gate location.
Long and thin metal articles previously have been cast with a directionally solidified or columnar grained crystallographic structure. When this is done, the entire mold is preheated to a relatively high temperature, which is substantially above the liquidus temperature of the metal. Superheated molten metal is then poured into the preheated mold. The mold is heated during the pouring process so that the metal is molten during and immediately after pouring. After the mold has been filled with molten metal, the molten metal is solidified upwardly in the mold cavity along a horizontal front. The casting of thin articles is described in U.S. patent application, Ser. No. 813,247, now U.S. Pat. No. 4,724,891, filed Dec. 24, 1985, by Ronald R. Brookes and entitled Thin Wall Casting. A general method of directionally solidifying a casting is described in U.S. Pat. No. 4,609,029.
Prasad's U.S. Pat. No. 4,809,764 (the '764 patent) entitled "Method of Casting a Metal Article," which was filed on Mar. 28, 1988, also describes a method for casting long and thin metal articles. The '764 patent is incorporated herein by reference. Although the '764 patent provides important information concerning casting nickel-chromium articles, it provides no information concerning other alloy compositions. This patent also specifically teaches that the rate of withdrawing the mold from the furnace during solidification need only be as slow as about 60 inches/hour to provide suitable, defect-free metal articles. More significantly, the '764 patent teaches preheating the upper portion of the mold to temperatures only at or slightly above the solidus temperature of the cast metal. There is no appreciation in the '764 patent of the particular benefits of heating the upper half of the mold close to the liquidus temperature of the metal composition.